1. Field of the Invention
The present invention relates to a thin-film magnetic head that reads and writes data signals, a head gimbal assembly (HGA) with the thin-film magnetic head and a magnetic disk drive apparatus with the HGA. Especially, the present invention relates to a thin-film magnetic head that writes data signals by a heat-assisted magnetic recording technique using a near-field light, an HGA with the thin-film magnetic head and a magnetic disk drive apparatus with the HGA.
2. Description of the Related Art
Recently, in a magnetic recording apparatus such as a magnetic disk drive apparatus, because its recording density becomes higher due to the spread use of data with larger volume, the thin-film magnetic head is strongly required to further improve its performance. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) effect element for reading data signals from a magnetic recording medium such as a magnetic disk and an electromagnetic coil element for writing data signals to the magnetic recording medium.
The magnetic recording medium has a magnetically discontinuous layer where magnetic microparticles are gathered together. Usually, each of the magnetic microparticles has a single magnetic-domain structure, and one recording bit consists of a plurality of the magnetic microparticles. Therefore, for improving the recording density, irregularity in the boundary of the recording bit is required to be reduced by decreasing the size (volume) of the magnetic microparticle. However, a problem is likely to occur that the size decrease causes thermal stability of the magnetization of the recording bit to be degraded.
A guide of the thermal stability of the magnetization is given as KUV/kBT, where KU is a magnetic anisotropy energy in the microparticle, V is a volume of a single microparticle, kB is Boltzmann constant and T is absolute temperature. Decreasing the size of the microparticle is equivalent to decreasing the volume V, thus, the thermal stability is degraded due to degrease in the KUV/KBT value. As a measure of the thermal stability problem, it may be possible that the KU is increased concurrently. However, the increase in the KU causes the increase in coercive force of the magnetic recording medium. On the other hand, a write field intensity of the magnetic head for writing data signals against the coercive force is limited by the amount of the saturation magnetic flux density of the soft-magnetic pole material of the head. Therefore, the head cannot write data signals to the medium when the coercive force exceeds the write field limit.
As the first method for solving the thermal stability problem, a perpendicular magnetic recording technique may be adopted instead of the conventional longitudinal magnetic recording technique. The thickness of the recording layer in the perpendicular magnetic recording medium can be increased more sufficiently than conventional. As a result, the thermal stability can be improved due to the larger volume V with the larger thickness.
As the second method, a patterned media may be considered as a candidate. While one recording bit consists of N pieces of the magnetic microparticles in the conventional magnetic recording as described above, one recording bit is a single pattern region with volume NV in the patterned media. As a result, the value of the guide of the thermal stability becomes KUNV/KBT, which means high improvement of the thermal stability.
As the third method for solving the thermal stability problem, a heat-assisted magnetic recording technique is proposed, in which the magnetic head writes data signals to the magnetic medium formed of a material with the large KU value by reducing the coercive force of the medium with heat supplied to the medium just before the write field is applied. The heat-assisted magnetic recording technique has some similarity to a magnetooptic recording technique, however, obtains a spatial resolution corresponding to a applied magnetic field region, while the magnetooptic recording technique obtain a spatial resolution corresponding to an emitted light spot.
As a proposed heat-assisted magnetic recording, Japanese patent Publication No. 2001-255254A describes a light recording technique utilizing a near-field light probe that has a metal scatterer with strobilus shape formed on a substrate and a dielectric material film formed around the metal scatterer. And Japanese patent Publication No. 10-162444A describes a technique in which a head provided with a solid immersion lens writes ultrafine domains on a magnetooptical disk using a micro light spot. Further, Japanese patent Publication No. 2000-173093A describes a structure in which a metal film with a pinhole is formed on an obliquely cut surface of an optic fiber. Further, U.S. Pat. No. 7,042,810 describes a heat-assisted technique in which an internal laser element emits a light to an optical fine aperture opposed to a medium. Further, Japanese patent Publication No. 2004-158067A describes a scatterer as a near-field light probe, which is formed in contact with the main magnetic pole of a head for a perpendicular magnetic recording in such a way that the irradiated surface of the scatterer is perpendicular to the surface of the medium. Furthermore, IEEE Transactions on Magnetics, Vol. 41, No. 10, pp. 2817-2821, 2005 describes a technique in which a recording pattern with the track width of approximately 70 nm is formed by using a near-field light and a magnetic field generated from a U-shaped near-field light probe formed on a quartz crystal slider.
In the above-described techniques, the method of heating the medium by using a near-field light generated from a near-field light probe or a scatterer which is irradiated with laser light is considered as a promising technique because a near-field light having a required intensity can be obtained with comparative ease.
However, there are some serious problems in these techniques. For example, in the technique described in Japanese patent Publication No. 2004-158067A, the light source is provided in a position much close to the head end surface, that is, much close to the recording medium so as to irradiate the scatterer with a light adequately. This configuration has a possibility that the light source may make contacts with the surface of the recording medium, thus is not preferable from the viewpoint of the apparatus reliability. On another front, this publication proposes a configuration in which the light source is distanced from the medium surface by using a mirror that changes the light direction by 90° (degrees). However, in the configuration, there occurs a problem that a light intensity loss may become larger due to the reflection at the mirror and the substantial elongation of the light path. Furthermore, this configuration of the structured element such as the mirror much close to the head end surface also has the problem associated with the apparatus reliability.
Further, the technique described in IEEE Transactions on Magnetics, Vol. 41, No. 10, pp. 2817-2821, 2005 enables the light to be provided under the condition that the light source is distanced from the medium surface without using any mirror. However, this technique is premised on the configuration in which the probe-formed surface of the head body is parallel with the opposed-to-medium surface of the head body. This configuration is quite different from that of the commonly used thin-film magnetic head in which the probe-formed surface (the element-formed surface) is perpendicular to the opposed-to-medium surface. Therefore, for example, it is highly difficult for this technique to be applied to the thin-film magnetic head for a perpendicular magnetic recording.
In addition, in the above-described techniques, there has occurred a problem that, in some cases, the recording layer of the medium may not be sufficiently heated by the near-field light. Actually, the near-field light exists only much close to the near-field light probe, the optical fine aperture or the scatterer. Then, the substantial existence region has a size of approximately the layer thickness, the tip width or the aperture diameter of the probe, the aperture or the scatterer. That is to say, the electric field intensity of the near-field light is rapidly attenuated from this existence region toward the medium. Therefore, even in the present situation that a flying height of the head is a greatly small value of 10 nm (nanometers) or less, the near-field light may not reach the recording layer of the medium sufficiently. As a result, a write error may occur because the coercive force of the recording layer is not reduced sufficiently during write operation.